Good day!

I created a webpage and it has a login page...I want that if the user was already login and she accidentally press the back button if the login page was view again she could not login again because she was already login..

I try to use session but i think it does not work. I have no idea on what code should i need to fix my problem.

here is my code:
PHP Code:
 <?php session_start(); //require_once 'conn.php';   $db_name="dspi";  mysql_connect("localhost", "root", "") or die("Cannot connect to server"); mysql_select_db("$db_name")or die("Cannot select DB");              $department = mysql_real_escape_string($_POST['department']);            $username = mysql_real_escape_string($_POST['username']);          $sql=mysql_query("SELECT `Department`, `Username` FROM `tbllogin` WHERE `Department` = '{$department}' AND Username = '{$username}'") or die(mysql_error());         $ct = mysql_num_rows($sql);               if($ct == 1) {             $row = mysql_fetch_assoc($sql);                         if($row['Department']=='Accounting') {                 header('location: Company.php');             } elseif($row['Department']=='Engineering') {                 header('location: Company.php');             } elseif($row['Department']=='Finishing_Goods') {                 header('location: Company.php');             } elseif($row['Department']=='HRAD') {                 header('location: Company.php');             } elseif($row['Department']=='MIS') {                 header('location:Company.php');             } elseif($row['Department']=='Packaging_and_Design') {                 header('location:Company.php');             } elseif($row['Department']=='Production') {                 header('location:Company.php');             } elseif($row['Department']=='Purchasing_Logistic') {                 header('location:Company.php');             } elseif($row['Department']=='QA_and_Technical') {                 header('location:Company.php');             } elseif($row['Department']=='Supply_Chain') {                 header('location:Company.php');             }             else {                 header('location:index.php');                 echo"Incorrect Username or Department";                                  }       } ?>  <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" ""> <html xmlns=""> <head> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" /> <title>DSPI LOGIN</title> <script>         function searchKeyPress(e)         {                 // look for window.event in case event isn't passed in                 if (window.event) { e = window.event; }                 if (e.keyCode == 13)                 {                         document.getElementById('submit').focus();                 }         }         </script> <style type="text/css"> <!-- BODY { background-image: url(layout_image/bgroundv09.png); background-attachment: fixed; } #Dept_Frame {     position:absolute;     width:229px;     height:49px;     z-index:1;     left: 441px;     top: 262px; } #Department_Option {     position:absolute;     width:186px;     height:32px;     z-index:2;     left: 453px;     top: 275px; } #Submit_Frame {     position:absolute;     width:82px;     height:35px;     z-index:3;     left: 516px;     top: 320px; } #Submit_Button {     position:absolute;     width:60px;     height:29px;     z-index:4;     left: 524px;     top: 328px; } #Username_ImageText {     position:absolute;     width:130px;     height:55px;     z-index:5;     left: 319px;     top: 208px; } #User_Frame {     position:absolute;     width:230px;     height:46px;     z-index:6;     left: 441px;     top: 216px; } #Username_Textbox {     position:absolute;     width:182px;     height:23px;     z-index:7;     left: 455px;     top: 228px; } --> </style>   </head> <body> <form id="form1" name="form1" method="post" action="">   <div id="Department_Option">     <select name="department" onkeypress="searchKeyPress(event);">       <option>Choose your Department. . . . . . </option>       <option value="Accounting" <?php if($_POST['department'] == 'Accounting') echo "selected='selected'"?>>Accounting</option>       <option value="Engineering" <?php if($_POST['department'] == 'Engineering') echo "selected='selected'"?>>Engineering</option>       <option value="Finishing_Goods" <?php if($_POST['department'] == 'Finishing_Goods') echo "selected='selected'"?>>Finishing Goods</option>       <option value="HRAD" <?php if($_POST['department'] == 'HRAD') echo "selected='selected'"?>>HRAD</option>       <option value="MIS" <?php if($_POST['department'] == 'MIS') echo "selected='selected'"?>>MIS</option>       <option value="Packaging_and_Design" <?php if($_POST['department'] == 'Packaging_and_Design') echo "selected='selected'"?>>Packaging and Design</option>       <option value="Production" <?php if($_POST['department'] == 'Production') echo "selected='selected'"?>>Production</option>       <option value="Purchasing_Logistic" <?php if($_POST['department'] == 'Purchasing_Logistic') echo "selected='selected'"?>>Purchasing and Logistics</option>       <option value="QA_and_Technical" <?php if($_POST['department'] == 'QA_and_Technical') echo "selected='selected'"?>>QA and Technical</option>       <option value="Supply_Chain" <?php if($_POST['department'] == 'Supply_Chain') echo "selected='selected'"?>>Supply Chain</option>     </select>   </div>   <div id="Submit_Button">     <input type="Submit" name="submit" value="Submit" id="submit" onclick="doSomething();"/>   </div>   <div id="Dept_Frame"><img src="layout_image/subframev02.png" width="229" height="50" /></div>   <div id="Submit_Frame"><img src="layout_image/subframev02.png" width="80" height="46" /></div>  <div id="Username_ImageText"><img src="layout_image/userv01.png" width="131" height="62" /></div> <div id="User_Frame"><img src="layout_image/subframev02.png" width="229" height="50" /></div> <div id="Username_Textbox">   <input name="username" type="text" size="30" /> </div> </form> </body> </html>